Cross Reference to Related Application
Commonly owned, concurrently filed U.S. patent application entitled "Combined Arc Shield/Wire Tray For Switchgear and Switchgear Assemblies Incorporating Same" and identified by Ser. No. 09/325,514.
1. Field of the Invention
This invention relates to switchgear used in low voltage electric power distribution systems, and more particularly, to an arrangement for routing and protecting control wiring within the switchgear cabinet.
2. Background Information
Switchgear for electric power distribution systems includes electrical switching apparatus and their line and load terminations together with related equipment mounted in metal cabinets. Switchgear used in sections of electric power distribution systems operating at voltages up through 600 volts is classified as low voltage switchgear. Typically, the electrical switching apparatus is a circuit breaker, but other switching apparatus such as for instance, network protectors, disconnect switches and transfer switches are also mounted in such switchgear cabinets. Henceforth, the electrical switching apparatus will be referred to as circuit breakers, although it will be understood that other types of electrical switching apparatus can be used as well.
Typically in such low voltage switchgear, multiple circuit breakers are mounted in each cabinet in cells stacked vertically in a forward compartment. The line and load conductors are mounted in rearward compartments and engage the circuit breakers through quick disconnects as the circuit breakers are installed in the cells.
Power circuit breakers can generate significant amounts of arc gases when interrupting large currents such as those associated with a short circuit in the distribution system. It is common for the arc gases to be vented through the rear of the circuit breaker. Other power circuit breakers discharge the gases through vents in the top of the circuit breaker housing. Generally, the approach is to slow the arc gases down and cool them before they leave the switchgear cabinet.
Other considerations in the design of low voltage switchgear is the placement of the terminals for the secondary wiring, such as the control and instrumentation wiring (normally at 120 volts). Generally, there is not much room for this secondary wiring terminations within the circuit breaker cabinet. One approach is to terminate the secondary wiring in the rear of the switchgear cabinet. However, this can expose personnel tending the secondary terminations to power conductors (normally at 480 or 600 volts) which are located there.
It is preferable to have the secondary wiring terminations at the front of the switchgear cabinet both for safety and accessibility. Hence, one approach has been to mount the secondary terminations alongside the circuit breakers. However, such space is very limited in the typical switchgear cabinet which is of standardized width.
Another approach has been to use an entire cell for terminations. Obviously this limits the number of circuit breakers that can be mounted in a switchgear cabinet. In some switchgear, the secondary terminations are mounted on trays which slide between the vertically mounted cells. In one arrangement, the trays pull out horizontally and then swing vertically for better access. One non-domestic manufacturer has fixed mountings for secondary terminations between the cells.
Placing the secondary terminations between the circuit breakers and accessible from the front of the cabinet must include protection of wiring from the arc gases discharged from top vents. The non-domestic manufacturer referred to above utilizes a rectangular channel extending transversely above the top vents to deflect the arc gases out sideways. The fixed secondary terminations are then mounted separately in front of the transverse channel.
The switchgear utilizing the pull out trays have a flat, horizontal arc shield at the top of the cells above the top vents. In some switchgear, this type of flat horizontal shield also extends under the pull out tray which is forward of the top vents. In this arrangement, where the pull out tray swings vertically after being pulled out, the flat wall is lower under the tray and there is an inclined transition section to this lower forward section. However, this is well in front of the arc vents. The switchgear also has flanges extending downward along the side edges of the flat ceiling panel.
The secondary or control wiring for low voltage switchgear assemblies includes factory-installed wiring which, in general, connects the circuit breakers with instrumentation also mounted in switchgear assembly and with each other for certain modes of operation. The field wiring, in general, is the wiring provided by the user of the switchgear for remote monitoring and control of the switchgear operation. Typically, the factory-installed wiring and the field-installed wiring interface at secondary wiring terminations. This factory-installed and field-installed secondary or control wiring should be easily accessible and protected from arcing gases. It should also not expose those servicing the secondary or control wiring to the power circuits within the switchgear assembly.
There is a need, therefore, for improved switchgear assembly, and especially for improved arrangement for routing and protecting the secondary or control wiring.